There is much advertising on chitosan (Poly-β-1,4-D-glucosamin) as a fat magnet. What is chitosan? Does it really work as described in the advertisements? Should we use it to avoid gaining weight? Students look for answers to these questions by researching the internet, and other sources, and doing own experiments. They learn how to produce chitosan, about its properties and several kinds of its application. The newly gained knowledge and competences form the base for the final discussion and students’ own decision making.
The Chitosan project can be taught as an advanced course deepening the knowledge on carbohydrates, but also extent it to more everyday life topics like healthy nutrition/ balanced diet with a focus on digestion and metabolism.
In the Schleswig-Holstein (Germany) chemistry curriculum the area 3 in grade 12/13 in the field of carbon hydrates refers to the topic handled here. It can also be used in the field of analytics (area 9). The sequence can suit each of the following topics:
- Carbon hydrates
- Structure-property interdependencies
Preparation of Chitosan
Generally, the isolation of chitin and chitosan from crab shells is possible (experiment 1 and 2).
If market products are used, cheap products of technical grade are recommended, because they are suitable for the experiments described below.
Analysis & Characterisation of Chitin & Chitosan
Like starch chitosan forms with iodine an inclusion complex, which has a purple colour in acidic medium. On the other hand, the chitin is unable to accommodate iodine molecules (experiment 3). In contrast to alkaline hydrolysis the glycosidic bonds of chitin between the sugar units are cleaved by acidic hydrolysis. The amide bond in chitin is preserved. Degradation of chitin leads to N-acetylglucosamine and corresponding oligomers. Hydrolysis of chitosan yields glucosamine and its oligomers (experiments 4 and 5):
The aldehyde groups of the glucosamine and N-acetylglucosamine are oxidized by Fehling's solution to carboxylic acids, whereas the divalent copper is reduced to univalent copper, which precipitates as brick-red colored copper(I) oxide in alkaline medium. The free aldehyde groups of chitosan are not sufficient for the Fehling's test (experiment 4), and. due to insolubility, the test with chitin is not feasible. The determination of free amino groups in chitosan is performed according to Slyke (Sommerfeld and Bader, 1995).
Application of Chitosan
The macromolecule chitin is used for the preparation of transparent films, because of its film-forming properties, which are caused to intramolecular and intermolecular hydrogen bonds. Therefore it is possible to use tetraethylene glycol as softening agent, which, by hydrogen bond formation, intercalates between the chitosan molecules (experiment 6).
Besides copper many other heavy metal ions like nickel, zinc, cobalt, iron(II), chromium(III) are chelated by chitosan. (Sommerfeld and Bader, 1995). The addition of very small amounts of chitosan to protein-containing waste water causes a agglomeration of the colloidally dissolved protein, as demonstrated in experiment 7. The Polycationic properties of chitosan decrease the electrical charge on the surface of the colloidally dissolved proteins and so allow the flocculation. The solutions treated show a negative biuret test and only a very faint Tyndall effect, indicating very small amounts of proteins.
A comparable effect is shown in experiment 8, where clarification of naturally cloudy juices is described. Juices contain naturally occurring polyanionic tannins and other suspended materials. Polycationic chitosan forms with those compounds ionic macromolecular complexes which are centrifuged off.
In alkaline medium chitosan reacts with monochloro acetic acid to N,0-carboxymethyl chitosan, experiment 9:
Like chitosan the carboxymethyl chitosan forms transparent films (experiment 10). Here the film-forming properties are also based on intramolecular and intermolecular hydrogen bond formation.
Experiment 11 shows very impressively, that a film made of N,0-carboxymethyl chitosan protects and conserves fruit. Without problems the coating is removed by washing before consumption. Fruits treated in this way keep well for more than three months when stored cool.
If chitosan is dissolved in hydrochloric acid, water soluble chitosan hydrochloride is formed (experiment 12):
When paper is treated with a solution of this hydrochloride, a film is formed on the surface, which becomes is water-repellent. This is nicely demonstrated with ink as described in experiment 13.